An organic photoconductor typically comprises an anodized layer or a barrier layer on a conductive substrate such as an aluminum drum, a charge generation layer (CGL) and a charge transport layer (CTL). The charge generation layer is made of a pigment dispersed in the binder layer. U.S. Pat. No. 6,033,816 to Luo et al. (patent '816) is illustrative of such photoconductors employing a blend of polymers as the CGL binder layer.
The use of polymer binder helps improve the dispersion stability and improve the adhesion of the CGL to the metal core. However, depending on the type of polymer binder being used, the sensitivity of the photoreceptor may be affected. Polymers typically used as binders of the CGL dispersions or solutions are polyvinylbutyrals, which may be blended with various resins such as phenoxy, epoxy resins, polycarbonates and polyacrylates. Such polymers may be inert to the electrical photographic properties. However, in some cases, the polymer may increase the sensitivity of the CGL (sensitivity being the extent of discharge of the charged electrical potential on a drum when exposed to a light source, typically a laser beam). The need to improve the sensitivity of a photoconductor is directly tied to the process speed of imaging with that photconductor. As speeds are increased and the laser optical power stays constant, less and less energy is delivered to the charged photoconductor.
With printers expected to perform at speeds in excess of 35 standard 81/2 in..times.11 in. pages-per-minute, it is necessary that the photoconductor charge and discharge at very short time intervals. The time frames required for such a 35 page-per-minute printer could relate to an exposed-to-develop time on the order of 50-80 ms. Hence, a growing need exists to identify systems that improve electrophotographic properties without compromising other properties such a adhesion and fatigue.
A second need is to obtain electrical uniformity of the photoconductor. The desire to have uniform print density across a printed image requires the photoconductor to have a low variance from end to end and around the drum. The uniformity of the electrical performance is tied to the uniformity of the coating and the homogeneity of the dispersion. Different polymer binders can help or be a detriment to the dispersion homogeneity.
Also, with long production runs and the need for stable pigment dispersions, if the dispersion changes properties so as not to support a full production run, the cost of each drum is increased. This is also true if the dispersion can not be stored between runs. Present dispersion systems do not have adequate shelf life or electrical uniformity to satisfy such objectives, leaving a need for a system which can deliver exceptional electrical properties, dispersion stability, and electrical uniformity.